The present invention generally relates to liquid ink recording devices using two or more different color inks. More particularly, the present invention is directed to reducing intercolor bleeding that occurs at the interface of areas printed with inks having different properties.
Liquid ink printers of the type often referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, employ at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
Liquid ink printers including ink jet printers deposit black and/or colored liquid inks which tend to spread when the ink is deposited on paper as a drop, spot, or dot. A problem of liquid ink printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desired. Bleeding tends to occur when the drops are placed next to each other in a consecutive order or in a cluster of dots within a short time. Bleeding, spreading, and feathering causes print quality degradation including color shift, reduction in edge sharpness and solid area mottle which includes density variations in said areas due to puddling of inks. Intercolor bleeding occurs when ink from one color area blends into or bleeds with ink from another color area. Intercolor bleeding is often most pronounced where an area of black ink (relatively slow drying) adjoins an area of color ink (relatively fast drying); however, intercolor bleeding can occur at the interface between areas of any color inks having substantially different properties such as dry time or permeability.
Various methods have been proposed to increase edge sharpness and to reduce intercolor bleeding. Some of the proposed methods include replacing slow drying black ink with a process or composite black formed by combing fast drying color inks; under-printing a portion of the slow drying black ink with a color ink, use a fast drying black ink, and using both fast dry and slow dry black ink. While all of the proposed methods reduce intercolor bleeding to some degree, they all have one or more drawbacks that effect printer performance and/or image quality.
For example, using a fast dry ink in place of a slow drying black ink results in a reduced quality of black reproduction as current fast drying black inks have lower image quality than slow drying black inks. Additionally, fast drying black inks typically result in fuzzy edges in black areas next to non-printed areas. The use fast drying black ink at an interface and slow drying black ink for interior regions can eliminate lower image quality associated with fast drying black inks, but increases the cost and complexity of printer design by requiring a fifth ink in addition to the cyan, magenta, yellow and slow drying black ink. Similarly, replacing slow drying black ink with a process black (composite black) generated from fast drying color inks typically results in a reduced quality of black reproduction resulting in a lower image quality than the use of slow drying black ink. Additionally, the use of process black increases the amount of ink deposited on the print medium, increases dry time and increase the time to print a document. Furthermore, the use of additional ink may not be suitable for print medium such as transparencies and some types of paper which is not very absorbent. Under-printing a portion of the slow drying black ink with a color ink can be used to reduce intercolor bleeding; however, under-printing increases the amount of ink on the print medium. Moreover, printing color under black often results in the thickening or blurring of edges particularly along edges between printed and non-printed areas.
One aspect of the present invention is a process for processing color image data for printing on an inkjet printer to reduce intercolor bleeding in an image on a receiving medium. The process includes the steps of: receiving color image data comprising a plurality of color planes that combine to form an array of composite pixels, the color planes including a black plane and a non-black plane, wherein each color plane comprises an array of separation pixels; identifying an interface between a black area and a color area; modifying the color image data corresponding to an M-pixel wide color border in the color area using a thinning operation according to a color pixel modification pattern, the color pixel modification pattern identifying a fraction of the separation pixels in a non-black plane to remove from the color image data; and modifying the color image data within an N-pixel wide black border in the black area using a substitution operation according to a black pixel modification pattern, the black pixel modification pattern replacing a fraction of the separation pixels in the black plane with separation pixels in a non-black plane.
A second aspect of the present invention is a method of processing color image data for printing on an inkjet printer to reduce intercolor bleeding in an image on a receiving medium that includes receiving a target pixel, the target pixel comprising a plurality of separation pixels each corresponding to an individual color plane, the color planes including a black plane and at least one non-black plane; determining if the target pixel is within an black border region near a black/color interface, and if so, modifying the target pixel using a substitution operation according to a black pixel modification pattern, the black pixel modification pattern identifying separation pixels in the black plane to replace with separation pixels from a non-black plane; and determining if the target pixel is within a color border region near a black/color interface, and if so, modifying the target pixel using a thinning operation according to a color pixel modification pattern, the color pixel modification pattern identifying a pattern of separation pixels in a non-black plane to remove from the composite image.
Another aspect of the present invention is a device for processing color image data reduce intercolor bleeding in an image on a receiving medium. The device includes a statistics collection filter connected to receive a target pixel and a set of surrounding pixels, the statistics collection filter generating a statistics signal; a pixel identification circuit connected to receive the statistics signal, the identification circuit generating a pixel identification signal indicating whether the target pixel is within a border region; and a pixel modification circuit connected to receive the pixel identification signal, the modification circuit modifying the target pixel according to a black bitmap modification pattern when the target pixel is within a black border and modifying the target pixel according to a color bitmap modification pattern when the target pixel is within a color border.